Oxoanions are a class of oxygen-bearing anionic molecules that can be formed during industrial processes. For example, typical oxoanions encountered during commercial processing operations include borate, carbonate, phosphate, sulfate, chromate, and arsenate. These oxoanions can be formed when a substance containing the non-oxygen element of the oxoanion is exposed to oxygen, water, and/or bacteria. Because of the adverse health and environmental effects associated with many oxoanions, governmental regulatory agencies often limit the level at which certain oxoanions can be discharged with waste water into the environment.
One situation where oxoanions can be formed is during the excavation of earthen material. Excavations such as mining operations, milling operations, road constructions, etc., can expose earthen material to oxygen and water, generating an aqueous effluent containing the oxoanions. These effluents can include, for example, acid mine rock drainage (referred to as ARD or AMD), mill tailings, excess decant water, seepages, and acidic process waste streams. Acid mine rock drainage forms when minerals in rocks are exposed to oxidizing conditions in mining operations, highway construction, and other large scale excavations.
Oxoanions can be difficult to separate from water because they often form relatively weak anions. Further complicating separation efforts is the recognition that oxoanion compositions and concentrations can change over time and with environmental factors. Factors such as rainfall, temperature, industrial process conditions, earthen matter content, and process chemical components can cause changes in the makeup of oxoanions in a given waste water stream over time. Ensuring that aqueous streams having variable oxoanion compositions and concentrations are properly treated can help ensure compliance with governmental regulatory requirements and good environmental and health stewardship.